Field
Exemplary embodiments of the present invention relate to a low-profile, thin side-lit, hollow cavity, LED backlight apparatus for LCD TVs, tablets, notebooks computers, and the like.
Discussion of the Background
Light emitting diodes (LEDs) have now become the preferred light sources for the backlight units (BLUs) of liquid crystal display (LCD) panels. As a consequence of their dramatic increase in efficacy, the number of LEDs required to adequately illuminate BLUs has decreased by a factor of 3-4 recently. Also consumer demand for thinner LCD panels in devices such as TVs mean that there may be a need for new optical architectures to allow for the manufacture of these thinner designs and yet still maintain adequate illuminance and display uniformity to the LCD panels.
Existing optics for direct-view BLUs consists of small refractive lenses placed directly over LED light sources, arranged in geometrical arrays, and acting as baffles to redirect the intense light coming from the LEDs, such that hot spots in the LCD panel may be eliminated or reduced to acceptable levels, with the result that the thickness of the resulting BLU may typically be 20-25 mm. For example, these thicker BLUs are disclosed in U.S. Patent Publication No. 2006/0203512A1, U.S. Pat. No. 7,293,899B2, and R. Tsuchiya et al. “Thin side-lit, hollow- cavity flat LED lighting panel for ultra-uniform LCD backlight applications” SID 08 Intl. Sym. Digest Tech Papers, 874-877 (2008).
A typical flat-panel display may utilize an LCD as a means of displaying information. LCDs, which allow the display of alphanumeric graphic or other information, comprise a medium whose transmittance changes in response to the application of electrical potential across the medium. LCDs may be viewed even in an otherwise dark environment by applying illumination uniformly to their rear face. An exemplary backlighting apparatus for an LCD is disclosed in U.S. Pat. No. 4,043,636.
Notebook computers and tablets having flat-panel displays may be used during travel and in areas with little light such as trains and airplanes. Hence, the LCDs included therein may have an acceptable level of brightness and uniformity so that the user can easily read the information on the display. Furthermore, software applications that are used in notebook computers, tablets, and LCD TVs usually require color images. Hence the display may have spectral content that is compatible with the colors displayed on the LCD. The light source may be compact so that it will not significantly increase the size and weight of the notebook computer, tablet, or LCD TV. Many LCD TVs are currently made in a 16×9 format, and the thinness of the display may be a critical factor to its purchase by an end-user. Most LCD TV backlight units may have a total thickness of 25 mm or less, and most recently, less than 13 mm thick.
Ultra-thin backlight units may be constructed using a 3-4 mm thick acrylic sheet, with the LEDs being placed along one edge of the sheet, to couple the light from the LEDs into a plastic waveguide and conduct the light throughout the sheet using total internal reflection (TIR). The light may be ejected using an out-coupling array of micro-dots or micro-lenses that disrupt the TIR light conduction and allow the light to be ejected uniformly over the back surface of the LCD display. However, the large acrylic sheets may be expensive and weigh too much. Thus, to make the backlight units less costly and lighter in weight, manufacturers have turned to so-called “direct view” backlights. Here, the large acrylic sheet is replaced by an array of small, light, and inexpensive lenses that baffle and spread the light emitted from the LEDs out over the surface of the backlight unit, such that it has sufficient illuminance and uniformity. However, as the thickness of the backlight unit decreases from 25 mm down to the 11-12 mm range, the refractive optical principles upon which these lenses are based may cease to yield acceptable uniformity.
U.S. Pat. No. 6,007,209 discloses baffling light emitted directly from LEDs to an LCD panel, such that the LEDs may be arranged into a planar array and then used as a direct-view BLU for an LCD display. However, at the time this patent issued, LEDs were not very efficient or powerful, but subsequently white LEDs have gone from an efficacy of about 5 lumens/Watt to more than 200 lumens/Watt. Thus, the baffling technology disclosed in U.S. Pat. No. 6,007,209 may provide a BLU with insufficient uniformity and illuminance value.